Electrical musical instrument



Aug. 7, 1951 J. M. HANERT 2,562,908

ELECTRICAL MUSICAL INSTRUMENT Filed April 16, 1949 3 Sheets-Sheet lWUILOENfxT- SNES .oJ/m

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anert 7W At Aug. 7, 1951 J. M. HANERT ELECTRICAL MUSICAL INSTRUMENT 3Sheets-Sheet 5 Filed April 16, 1949 e000@ OMNU Inventor John M. Ha

Patented Aug. 7, 1951 UNITED STATES PATENT OFFICE ELECTRICAL MUSICALINSTRUMENT John M. Hanert, Park Ridge, Ill., assigner to HammondInstrument Company, Chicago, Ill., a corporation of Delaware ApplicationApril 16, 1949, Serial No. 87,913

(Cl. Sli-1.22)

11 Claims. 1

The invention relates generally to electrical musical instruments andmore particularly to means for improving the quality of the tonesproduced by an instrument of this type.

It is the primary object of the invention to provide an electricalmusical instrument having improved means for producing electricalsignals corresponding to musical tones of highly desirable tone quality.

A further object is to provide an improved musical instrument having amaster oscillator and a plurality of alternate pulse responsivefrequency divider stages, to derive from `the oscillator and thefrequency divider stages sharp pulse signal waves, and to provide meanswhereby such pulse waves are converted into electrical signals of waveshapes representing highly desirable musical tone qualities.

-A further object is to provide an improved tone signal generatingsystem for electrical musical instruments in which alternate pulseresponsive frequency divider stages are employed, and in which anelectronic rectier is used to provide sharp peak pulse signals suitablefor reliable operation of the frequency divider stages, such sharp peaksignals being also employed to provide the musical tone signalscontaining a long series of both the odd harmonics and the even harmonicpartials.

Other objects will appear from the following description, referencebeing had to the accompanying drawings in which:

Figs. 1 and 1a together constitute a schematic wiring and block diagramof an electrical musical instrument incorporating the invention;

Fig. 2 is a chart of curves showing the harmonic content of tone signalsof various frequencies produced by the instrument;

Fig. 3 is a chart similar to Fig. 2 showing the effect of the takingofi' the signal at various points in the attenuating mesh utilized in.the instrument;

Fig. 4 is a schematic wiring diagram showing the ltering mesh from whichthe data used to plot the curves of Fig. 3 were obtained; and, Fig. 5shows the wave form of the input signal to the circuit of Fig. 4 andwhich was used to obtain the data for the plotting the curves of Fig. 3.I

In electrical musical instruments, particularly of the solo o-r melodytype, in which a `master oscillator has its frequency controlled byplaying keys or the like, and in which frequency dividers arevcoupled incascade to the masteroscillator, it is highly' desirable that thefrequency dividers stages be capable of operating through a wide rangeof frequencies and that they have a high degree of stability so asalways to produce signal frequencies which are the desired submultiplesof the frequencies to which the master oscillator is tuned. Frequencydividers having these qualiiications usually require a sharp pulse typeof input signal to insure reliable operation, and have an output inwhich the wave shape is rectangular or of saw-tooth shape.

A saw-tooth wave is one in which the harmonies are of geometricallydecreasing amplitude, in which, assuming the intensity of thefundamental to have a value of 1, the successive harmonics will haveintensities of 1/, 1/3, 1A, 1/5, etc. Such saw-tooth wave is musicallyunsatisfactory for two reasons: First, the rate of progressiveattenuation of the low order harmonies (2nd to 5th) is too rapid. Forexample, the second harmonic is attenuated approximately 6 db (decibels)with respect to the fundamental. The other low order harmonics arecorrespondingly attenuated in relatively large steps. Such highattenuation of the low order harmonies relative to the intensity of thefundamental, results in the production of excessively pure musical tonessince too much of the energy is concentrated in the fundamentalfrequency. The ear is more pleasantly stimulated by the reception of atone in which the energy is more or less equally distributed over areasonv able number of lo-w order harmonics.

A second reason that tones having a harmonicA series as represented by asaw-tooth wave are not of a satisfactory quality is that the higherorder harmonics are of substantially equal amplitude. This is wellillustrated by the curve S in Fig. 3, in which the intensities of thepartials of a saw-tooth wave are indicated. y It will be noted that theright hand portion of the curve tends to become nearly horizontal, thatis, the higher harmonics do not differ substan-. tially in intensity.This latter condition is musically undesirable since it causes the toneto has the same harmonic analysis as the saw-toothv wave except that noeven harmonics are present.4v

The musical tones represented by rectangular or square wave shapes thushave the same deficiency as the saw-tooth wave in that the attenuationof successive low order harmonics is at a too rapid rate, while the rateof attenuation of successive higher order harmonics is at too low arate.

A tone quality which is incalculably superior to thatwhich' Ycan beattained by the use 'of a saw-tooth or a rectangular wave shape signal,is one in which each of the harmonic partials is attenuated a constantpercentage relative to the preceding harmonic. With the apparatus ofthis invention, the rectangular Wave output of the frequency divider isrectified to a very narrow pulse whose width is small in comparison withits period. All of the low order harmonics are then present insubstantially equal amplitude. This equality of harmonic amplitude willrcontinue to higher harmonics only if the pulse is sufficiently narrow.For musical purposes this initial quality of amplitude is desirable onlyfor the lower order harmonics (such as below the flfth harmonic). Thenarrow pulse wave is extremely rich in high harmonics and generally of aquality far too bright forfmusical purposes. it is a wave which is toobuzzy but nevertheless, it is not too Ypurei in its low order harmonicenergy distribution. Thus, the narrow impulse wave satisfies thefirstrequirement of an ideal toneinsofar as the low order harmonics areconcerned. In order to attenuate the higher harmonics sufficiently, amulti-stage filter is provided Which is eifectiveto cause very greatattenuation of the undesirable high harmonics without correspondinglyVaffecting the fundamental and low order harmonic relations. By properselection of the` frequency characteristics of Ya multi-stage lternetwork, a substantially ideal tone may be obtained in which the 40thharmonic is attenuated 30 decibels. This corresponds to a 1.2 decibelper harmonic attenuation factor. When listened to, this is a verypleasing rich tone-quality corresponding to a carefully voiced cello inwhich there is no suggestion of unpleasant low order harmonic purityvorhigh order harmonic buza As the musician plays upA and down the keyboardthe ideal tone is everywhere present. Furthermore, there is somevariation in the slopes of the straight-line harmonic curves (see Fig.3). The tones are more brilliant at the lower end of the keyboard andmore mellow at the high-pitched end of the keyboard. This is desirablynatural sounding, and corresponds to the reduction in brillianceoccurring in fine pianos as the player ascends the scale. A smoothvariation in tone quality over the compass of the keyboard is desirablebecause it believes tonal monotony and adds interest to the music beingperformed.

l Curves indicating the harmonic analysis of such tones are shown inFig. 2, and will be referred to hereinafter.

The invention is herein illustrated as forming part of an electricalmusical instrument of the solo or melodytype such, for example, as isshown in the Patent Nos. 2,203,569 and 2,233,258. Certain features ofthe oscillator and other parts of the instrument are shown and claimedin my copending4 application Serial No. 51,409 filed September 27, 1948.

The solo oscillator may be of any suitable construction but is hereinillustrated as comprising triodes 24 and `25 having a resonant tuningcircuit including a capacitor C23 connected to a,

terminal 22 and hence to the grid of triode 24, and playing keycontrolled timing apparatus l5, comprising the customary manual ofplaying keys, approximately three octaves long, which progressivelyconnect suitable inductance elements in series between terminal 22 andground. In addition, each of the playing keys completes a circuitVbetween a conductor I5 and ground to control the transmission of thesignal through the output system of the instrument.

The oscillator 24, 25 may include adjustable tuning controls IBconnected between ground and the terminal 22, and a vibrato apparatus 20similarly connected between ground and terminal 22 to cause a periodicshift in the frequency of oscillation of the oscillator 24 through arange of approximately 3 percent and at a rate of from 5 to 7 cycles persecond.

'.Triodes 24 and 25 are connected to ground through self bias resistorsR58 and R29 respectively. The anodes of triodes 24 and 25 are connectedto a suitable source of plate current, indictated as +285 v. terminal,through load resistors R55 and R52 respectively. The anode of triode 24is connected to the grid of triode 25 through a blocking capacitor C54and a series grid resistor R55, the junction between C54 and R56 beingconnected to ground through the grid return resistor R58. The anode oftriode 25 is connected to the grid of triode 24 through a capacitor 55in series with a resistor R52 to provide a feed back path. Thisoscillator, due in part to the provision of the series grid resistorR55, produces Van ouputV Wave on the cathode of triode 25 which isnearly rectangular in shape. A wave of `this character consistssubstantially exclusively of a fundamental and a long series of its oddharmonics.

In operation, the triode 25 functions in th manner of either a class Aor class C amplifier (in neither case drawing grid current) and thus thesignal appearing on the grid terminal 22- appears in opposite phase onits plate. Triode 25 operates in a non-linear manner to cut off itsplate current when its relativelylarge grid signal is` negative andfurther limits its plate current when its grid signal is positive. Thepositive limiting effect is made possible because of the provision ofthe series grid resistor R55. This resistor-R56 functions to preventfurther increase in signal amplitude at` the grid itself when any gridcurrent is drawn. This isl because the internal input impedance of thetriode 25 becomes very low in comparison with R56 when the grid becomespositive with respect to its cathode. Furthermore, resistor R55functions to prevent the capacitor C55 from building upY a negative biaswhich would thereby cause triode 25 to operate in a class C manner toproduce positive pulses at its output instead of the rectangular wavewhich is musically desirable because of its content of a series of Aoddharmonies.

Y The series resistor R56 is made large in comparison with the shuntresistor R58 so that the charge on capacitor C54 is substantiallyunaffected by the grid rectification in triode 25. A still furtheradvantageof including resistor R55 is that changes in the voltage of thesignal at the anode of triode 24 are not effective to change the D. C.voltage across the capacitor C54, There-V fore capacitor C54 can be maderelatively large so as to prevent any undesirable phase shift within theoscillator frequency range, and no diiliculty with cessation of.oscillations is encountered even if there are large changes in theimpedance of the tuned mesh. Suc-h changes would result in largeamplitude changes on the plate of tube 24, which in turn would changethe charge across condenser capacitor C54, were the resistor R56omitted.

Therefore, the inclusion of resistor R56 serves not only to cause tube25 to operate in a desirable non-linear manner, but also prevents thedevelopment of a self-bias across the relatively large capacitor C54,which bias wauld cause stopping of the oscillator when playing legatofrom a note at which the resonant mesh impedance is high (therebytending to cause a large self-bias voltage to be developed) to a note atwhich the resonant mesh impedance is relatively low. If this were tohappen the change in the bias voltage across capacitor C54 would be ofsuch value as to cut off the triode 25 when first playing the secondnote, thereby causing cessation of oscillation for a period related tothe time constant of C54 and R58. As explained above this time constantshould be long in comparison with the periodicity of the lowest note, sothat no undesirable phase shift will occur. By inclusion of R56 thistime constant problem is obviated and the oscillator does not stoposcillating. In a particular circuit which I have found satisfactory thetriode 2,4 may be one-half of 'a 6SL7 double triode tube, and the triode25 may be one-half lof a 6SN7 double triode tube. The value of R56,under these circumstances is .1 megohm, and R58 is .05 megohm.

The rectangular signal appearing across the cathode resistor R49 isutilized in the output of the instrument, being transmitted through aswitch SI, a blocking (and low frequency attenuating) capacitor C64 anda conductor 3|. The switch SI forms one of a group of 8 switches SI toS8, the odd numbered switches being adapted to make contact with an oddharmonic bus-bar 1|, while the even numbered switches S2, S4, S6, and S8are adapted to make a connection with an odd and even harmonic bus-bar12. The switches are operated in groups, SI and S2 being adapted to bemanually operated together by any suitable control, S3 and S4, S5 andS6, and S1 and S8 being similarly operated together, as indicated by thedotted lines in Fig. 1.

The plate of triode 25 is coupled through a blocking capacitor C98 and aseries grid resistor R99 with the grid of a pulse sharpening andrectifying triode 96. The junction of the capacitor C98 and R99 isconnected to the cathode by grid resistor R| and the cathode isconnected to ground through a signal resistor R|0|. Due to the provisionof a small current limiting condenser C-98, the substantially squarewave signal appearing on the plate of triode 25 is changed to a signalhaving acute positive and negative peaks symmetrically disposed aboutthe horizontal axis. In as much as triode 96 is operating with zerobias, the negative signal peak is effective to out the tube off (therebyproducing a positive signal pulse in signal resistor RIOI). However theseries grid resistor R99 andthe lack of bias prevents the positivesignal peak from causing substantial plate current change. Thisrectifying action results in a signal being generated across RIOI whichcontains even as well as odd harmonics. In addition the wave-fronts ofthis rectified wave are steep and well-suited for operation `of thesucceeding frequency divider stages.

Plate voltage for the triode 96 is supplied from a +285 v. terminalthrough a load resistor RI06,

, put 0f the third divider Stage.

6 andthe plate of triode 96 is connected tothe grid of a. rectifiertriode IIO through a coupling capacitor CI 04 and resistor RI 4.

The normally cut-off triode I |0 operates to further rectify the signalso as to provide sharp negative pulses for the operation of the firstfrequency divider stage which comprises triodes |20 and |2I. Platevoltage is supplied to the triode I|0 through a load resistor RI24 froma suitable potential source indicated +285 v. The pulse signal appearingon the plate of the triode |I0 is impressed upon the grids of triodes|20 and |2I, and these grids are connected to ground through gridresistors RI34. The grids of triode |20 and I2| lare respectivelyconnected to the plates of triode |2| and |20 through meshes, eachcomprising a series resistor R|36 shunted by a capacitor C|40 and aresistor RI38 in series. Plate voltage is supplied to the triode |20through a load resistor R|42 which is connected to a +285 v. terminalthrough a mesh comprising a resistor RI56 having a capacitor CI51 inparallel therewith. The junction of R|42 and R|56 has a conductor 33connected thereto for transmitting a signal to the output system of theinstrument. Plate voltage is supplied to the triode I 2| through a loadresistor RI 54 connected to a +285 v. terminal.

The output signal appearing on the plate of triode I2| is impressed uponthe grid of triode |86 (Fig. 1a) through a coupling capacitor C|88, thegrid of triode |86 being connected to ground through a resistor RI34.Triode |86 operates as a rectifier to supply very sharp, narrow,negative pu-lses to triodes |90 and |9I which form parts of the secondfrequency divider stage. This divider stage, comprising the rectifiertube |86 and triodes |90 .and 9|, is similar to the first frequencydividerustage and corresponding components have therefore had the samereference characters supplied thereto, it being understood that thecomponents function in the same manner but that they may have doubledvalues to insure more reliable operation at the sub-octave frequency atwhich the second frequency divider stage operates.

Similarly, a third divider stage is similar to the first frequencydivider stage and comprises a rectifier triode I 94 and a pair ofalternate pulse responsive triodes |96 and |91. The plate of the triode|91 is connected by a coupling capacitor C|98 to the grid of a pulsesharpening triode |99. This triode is included so as to provide a sharppeaked pulse signal for the out- This signal is derived across Aa meshRISE, CI51 in the plate circuit of triode |99 and is transmitted througha conductor 38 in a manner similar to that in which the signal isderived from the plate of triode |20.

It will be noted that signals are also taken from the plate circuit oftriode |86 through a conductor 34, from the plate of triode |90 througha conductor 35, from the plate circuit of triode |94 through a conductor36, and from the plate circuit of triode |96 through a conductor 31.

The cathodes of triodes ||0, |20, I2I, |96, |90, |9I, |94, |96, |91, and|99 are connected to al cut-off'bias by means of a common conductor 202which is biased to ground potential by a self-bias resistor R203 (Fig.la) having a capacitor C204 in parallel therewith.

It will be `clear from the foregoing that each of the' odd numberedconductors 3| to 31 is adapted to transmit a square or box wave signal,

whereas the even numbered .conductors 32 to 38 are connected so as toreceiveaV sharp, narrow, needle-like, negative pulse signals from theirrespective divider stages.

As Apreviously stated, the rectangular, clarinetlike, signal appearingacross cathode resistor R49 is supplied to switch Sl through a small`current-limiting capacitor C64. This capacitor thus serves to equalizethe low order harmonics. The string-like signal (having both even andodd harmonics) appearing across the cathode resister RIG! is impressedthrough conductor 32 and. decoupling resistor R2I2 upon the switch Theconductors 3.3 to 38 inclusive are connected to their respectiveswitches vS3 to S8 through suitable ltering meshes which include smallcurrent-limiting capacitors C203 to `C208 and decoupling resistors R2|3to R2l8 respectively. These small condensers also `serve to equalize theamplitudes of the fundamental and low order harmonics. The filteringmesh for attenuating the higher order harmonics in the contralto rangeodd harmonic signal supplied by the conductor 33 includes a seriesresistor R220 the terminals of which are connected to ground throughR221 and C222 respectively. R22! is of high value such as l megohm andserves to maintain the switch contact 53 at ground potential, and thusprevents possible transients when closing switch 53. In a similar mannerthe attenuating mesh for the sharp pulse signal supplied by theconductor 34, that is, the contralto odd and even harmonic signal, in-

cludes series resistors R224 and R225, shunt resistor R226, and shuntcapacitors C221 and C228. The tenor odd harmonic signals suppliedthrough the conductor 35 have their higher harmonics attenuated by thefiltering mesh comprising series resistors R23l, R232 and R233, shuntresistor R234 vand -shunt capacitors C235, C236 and C231.

The tenor pulse signals containing the odd and even harmonics, suppliedthrough conductor .36, .have their higher harmonics attenuated by thefiltering mesh comprising series resistors R260, R241, R242 and R223 andshunt resistance R244 and shunt capacitorsl C245, C246, C241 and Thebass signals containing .only the odd harmonics, supplied through theconductor 31, have their higher order frequencies attenuated by theiltering mesh comprising series resistor R25l, shunt resistor R252 andshunt capacitor C253.

The bass odd and even harmonic signals, supplied through the conductor38, have their higher harmonics attenuated by the filtering `meshcomprising series resistors R260, R261, R262, shunt resistor R263 andshunt capacitors C252, C265 and C266.

Whether the signal is to comprise only tones having odd harmonicpartials or tones having both the odd and even harmonic partials isdetermined by the position of a switch 210 which is adapted to connect aconductor 21| to either of the bus conductors 1|, 12. The conductor 21!Afeeds the selected signals to a preamplier 212 and the latter suppliesthe amplified signals to a suitable envelope and tone control apparatus214 from which the signals are supplied to a volume or expressioncontrol device 216, a power ampliiier 218 and'speaker 280.

It will be recalled that the playing key con-V trolled tuning apparatusl5 not .only includes means controlled by the playing keys to tune thesolo or master oscillator but also includes a switch under each keyadapted to connect conductor I6 to ground. Conductor I6 leads to theenvelope control apparatus 21.4 and, by operating on the bias of vacuumtubes included in the apparatus 14, renders this apparatus capable oftransmitting the signal from the preamplifier to the volume orexpression control 216, in a manner disclosed generally in the afore-`said Patent No. 2,233,258.

The resultsfattained by the use of the filtering meshes for the outputsof the various frequency signal sources may best be understood byreference to Figs. 2 to 5. Fig. 4 illustrates a ltering mesh similar tothat connected to conductor 36 and which was used to obtain readingsfrom which most of the curves of Fig. 3 were plotted. A signalcomprising a pair of positive and negative peaks, symmetrical about thezero-axis was supplied to the circuit of Fig. 4 through a blockingcapacitor C290 connected to the grid of a trdcde 232, the grid beingconnected to the ground through a resistor R294 The cathode wasconnected to a suitable potential source, such as +50 v., Ato providethe desirable grid cut-off bias. Plate current was supplied to this tubefrom a suitable potential source B+ through voltage divider loadresistors R236 and R291. The signal appearing at the junction of thelatter two resistors, that is, at the point a in Fig. 4, had the waveshape of Fig. 5, and was attenuated by a filtering mesh comprisingseries capacitor C298 and series resistors R290, R300, R30l and R322.The junctions following the latter four resistors bear the referencecharacters b, c, d, and e, and `are connected to ground throughcapacitors C306 to C309 respectively, while the junction between C28 andR299 is connected to ground through a resistor R3l0.

In Fig. 4 the various components have their values (in ohms andmicrofarads) indicated in the parenthesis following the referencecharacter. above was impressed upon the grid of the triode 292 andharmonic analyses made of the signals appearing at the terminals a, b,c, d and e and the intensities the harmonics plotted to produce curvesA, B, C, D and E respectively of Fig. 3. (It will be understood thatthese curves designate merely the intensities of diierent harmonics, anddo not represent continuous variables.)

In Fig. 3 the intensity is plotted vertically to a logarithmic scalewhile the harmonic numbers extend along the horizontal axis. From thesecurves-it will be noted that very desirable musical tone qualities areproduced by the Vattenuating meshes of Fig. 4 which correspond to themeshes by which the generators are coupled to the output of theinstrument. Each of the curves A to E is substantially a straight-line,which means that the successive harmonic.l partials are attenuatedprogressively by equal percentages. The frequency ci the signal suppliedto the network shown in Fig. 4 was that of the note CI having afrequency of 128 C. P. S. With a note of this frequency it is desirablethat the number of harmonics be very large since many of the harmonicslie well within the range of audibility. These curves show how the tonequality of the signal is progressively changed as it is attenuated bythe successive sections of the iiltering mesh of Fig. 4.

As shown by curve E, the higher harmonics are of suiiciently lesserintensity than the lower o1- A pulse type signal of the Aform described'r`derharmonics that the tone does not sound buzzy. For higher pitchnotesit is not desirable kto have the slope of the curve representing theintensities ofthe harmonics be as gradual as the slope of the curve E ofFig. 3.

This fact is illustrated by the curves in Fig. 2..-wherein the curve CIcorresponds to the curve E ofFig. 3. This curve represents the intensityof the various partials of the note of a pitch CI, that is having afundamental frequency of 128 C.-P. S. Similarly, the curve C2 representsthe intensities of thel partials of the note C2 having a fundamentalfrequency of 256 C. P. S. It will be noted that the harmonics of thenote C2 decrease in intensity at a substantially more rapid rate thanthat of the note CI. Likewise, the curves C3 and C4 representing theintensities of the partials of the notes C3 and C4, respectively of 512C. P. S. and 1024 C..P. S., are of progressively greater slope than thecurve C2. The note C4 is quite high in the musical register and if itcontained audible'frequencies much above the sixth or seventh harmonicthe tone would sound disagreeable since it would be extremely buzzy.

Theconstants of the filtering meshes for atktenuating the higher orderharmonics of the signal supplied through the conductors 33 to 38inclusive, are such asto cause the attenuation to conform generally tothat represented by the curves of Fig2. By this method of toneproduction, whereby a signal containing practically all of the audibleharmonics in substantially equal amplitude, has its higher orderharmonics attenuated progressively, by a substantially constant factor,as their harmonic number increases, musical tones of very desirablebasic quality are obtained.

These tone signalsare then passed through the usual adjustable filteringR-L-C meshes whereby certain tonal ranges are accentuated due to theresonance eifects ofl these circuits, to produce a large variety ofdifferent tone qualities. It will be understood that such resonantnetworks are much more effective if the signal supplied to it .includesa large number of harmonics of substantial amplitude than if the signalincludes but a few harmonics of audible amplitude. These flteringtonecontrol networks are much more effective on a signal having a wave-shapesimilar to that shown in Fig. 5 than on a signal of the saw-tooth waveshape, of which the harmonic analysis is illustrated by the curve S inFig. 3. For example, if a saw-tooth wave having the characteristics' ofthe curve S of Fig. 3 were passed through a filter tending to pass aband of frequencies centering around the fifth harmonic, it will beIclear that the first and second, and possibly the third, harmonicscould not readily be attenuated sufficiently to be of lower amplitudethan the fth harmonic whereas the harmonies of higher order than thefifth would be greatly attenuated. On the other hand, if the signalrepresented by curves C, D or E were lilvtered by such a band passfilter, the lower order n produced by the generating systems disclosedin 7 ,this application are more readily susceptible to .control as toquality by band pass filtering meshes. While it will be apparent tothose skilled in the art that the particular values of the components lof the'lte'rin'g circuits may be varied considerably and yet producesubstantially the same results, it is believed desirable to set forththe values of a number of the important components of filtering circuitswhich have been found to be satisfactory for the accomplishment of thepurposes set forth as objects of this invention. The values may bevaried considerably, especially if the compensatory changes are made inthe values of other components. In the particular circuits shown in thisapplication the various important components have the following values,in ohms and microfarads:

C64, .0001 C245, 004 27 M R240, 22 K C203, .001 C246, .002 R217, .15MR241. 39 K C204, 0l C247, 001 R218, .15 M R242. 82 K C205, 0025 C248,0005 R220, 39 K R243, 16 M C206, .02 C263, 003 R221, 22 K R244, 1 MC207, 005 C264, 004 R224. 39 K R251, 39 K C208. .04 C265, 002 R225, 82 KR252, 22 K C227, 001 C266, 001 R226, 1 M R260, 39 K C228, 0005 R212, 47M R231. 39 K R261, 83 K C235, 0008 R213, 27 N. R232, 82 K R262, 15 MC236, 0004 R214, 39 M R233, 15 M R263, 1 M C237, 0002 R215, 1 M R234. 22K The capacitors CI51 may have values in the order of .005 to .01, whilethe resistors R156 may have values in the order of l0 K to 118 K,depending somewhat upon the signal strength of the output of thefrequency divider stages to which they are connected.

In the foregoing description, and in the following claims, the termalternate pulse responsive frequency divider is used to refer to anyfrequency dividing device, which has two, and only two, stable outputvoltages, and which will continue to deliver one of these voltages untila controlling input pulse is applied thereto, whereupon the other outputvoltage will be delivered. Thus, upon application of successivecontrolling impulses, the device will deliver the two output voltagesalternately, and the output voltages alternately, and the output wavewill be generally rectangular in shape and have a frequency one-halfthat of the controlling input pulses.

While I have shown and described particular embodiments of my invention,it will be apparent to those skilled in the art that numerousmodifications and variations may be made in the form and constructionthereof, without departing from the more fundamental principles of theinvention. I therefore desire, by the following claims, to includewithin the scope of my invention all such similar and modified forms ofthe apparatus disclosed, by which substantially the results of theinvention may be obtained by substantially the same or equivalent means.

I claim:

1. In an electrical musical instrument of the melody type, having anoutput system, the combination of a variable frequency masteroscillator, keyboard controlled means for tuning the master oscillatorover a range of at least two octaves in accordance with the intervals ofthe tempered musical scale, an alternate pulse responsive frequencydivider coupled to said master oscillator and supplying a substantiallyrectangular shaped output signal of one-half the frequency of the masteroscillator, a high pass lter network coupled to said frequency dividergreatly to attenuate the fundamental and low order harmonics of theoutput signal and effective to alter its wave from a rectangular shapeto one in which there 0 are two peaks substantially equally spaced intime but oppositely polarized for each cycle of the signal, a rectifiercoupled to said network and effective to attenuate one of said peaksrelative to the other and thereby to introduce musically desirable evenharmonics into the signal, an output ,terminal for said rectier,alow-pass filtering network coupled to said terminal for attenuatingundesirable high harmonics in the signal present "at such outputterminal, a second alternate pulse Y vresponsive frequency dividercoupled to said output terminal and effective to divide the frequencythereof by a factor lof two, and means for transmitting the outputsignal from said low pass filtering network to the output system of theinstrument.

2. In an electrical musical instrument having an output system and aplurality of alternate pulse responsive frequency divider stages eachoperable throughout 'a wide range of musical frequenciesin responsetovariable frequency input signal pulses, and each producing a rectangularwave output signal comprising a fundamental frequency and a long seriesofv odd harmonics thereof; the combination of means for coupling theoutput of one stage to the input of the next stage comprising a currentlimiting and rectifying device effective to change the rectangular wavein the output of the first stage into a 'sharply peaked wave in whichthe peaks of one polarity are greatly attenuated, and a multistage tothe output system and effective to attenuate the harmonics progressivelyto greater extents rwith increasing frequency by a substantiallyconstant fractional factor.

3. In an electrical musical instrument having an output system and aplurality of alternate pulse responsive frequency divider stages eachoperable throughout a wide range of musical' frequencies in synchronismwith variable frequency input signal pulses, and each producing arectangular wave output signal comprising a vfundamental frequency andva long series of odd harmonics thereof of nearly equal amplitude;

means for coupling the output of one stage to the input ,of the nextstage comprising a current limiting and rectifyingl device effective tochange the rectangular wave in the output of the first stage into asharply peaked wave in which the peaks of rone polarity are greatlyattenuated, and imulti-section lt'ers respectively coupling the outputof the coupling means and the outputs of the divider Stages to theoutput system of the instrument, said filters being constructed andarranged to attenuate the harmonics of the sig- 1 nals to an extent thatsuch each harmonic has an intensity which is a predetermined yfractionof that of the preceding lower harmonic.

4. In an electrical musical instrument 'the combination Yof a musicalfrequency signal source, an electron discharge device having a currentlimiting input'circuit operable to peak the signal supplied by thesource and having an output circuit in which the signal appears as awaver comprising a series of sharp peaks of one polarity spaced*- farapart along the time axis, an output system Vfor the instrument, and acircuit coupling the output of the electron discharge device to theoutput system, said circuit including a multisection filter havinghigh-frequency attenuating-r e5 Y filter coupling the output of thecoupling means in theaudible range in nearly vrequal high-amplitude,attenuating the v`harmonics of the lower lfrequency-waves Vso that theintensity of each harmonic partial isalredetermined large fraction ofthe intensity of the next lower partial, and attenuating the higherfrequencywaves sothat the intensity of each harmonic partial is apredetermined lesser fraction of the intensity of the nextlower partial.Y v

6. In an electrical musical' instrument having an output system, thecombination of va Vseries ofY cascaded alternate pulse responsivefrequency divider stages, means to supply a controlling frequency to therst divider stage of the series, a plurality of peaking and rectifyingdevices respectively' forming coupling means between each divider stageand the succeeding divider stage of the series, and a plurality ofselectively operable means respectively'couplingthe outputs of theindividual speaking and rectifying devices and the4 individual dividerstages to the output system, said last ynamed means including lter meanseffective to attenuate the higher order harmonics.

7. The combination set forth in claim 6 in which the filter means forvrthe tone signals attenuates the partials of the signals approximately indirect proportion to the frequencies of the partials.

8. In anelectrical musical instrument having an output system, theYcombination of a master oscillator, a first alternate pulse responsivefrequency divider coupled to said master oscillator and effective todivide its frequency by a factor of two, a rectier coupled to the outputof said first frequency divider, an output terminal for said rectifier,a second alternate-pulse-responsive lfrequency divider coupled Vto saidterminal, and

transmitting sharppulses of alternating polaritycomprisingrsubstantially only odd harmonic partials to theoutput'syste'm, and a second signal transmission path includingpulse-forming means and rectifying means connecting the source With theoutput system andtransmitting sharp pulses of only one polaritycomprising both even and odd harmonic partials.

10. In an electrical musical instrument having an output system, thecombination of a source of electrical musical tone signals, meanscoupled to the source for producing symmetrical rectangular wavesignals, current limiting means connected to the output of the lastnamed meansfor producing substantially needle sharp pulses ofalternating polarity, rectifying means connected with the output of thecurrent limiting means for transmitting pulses of one polarity only, anda filter connecting vthe rectifying means with the output 'systemattenuating the harmonics of the pulses approxinfiatelyV indirectproportion to the frequencies of 'thehar'monics 11. In an electricalmusical instrument having aiseasoe n output system. the combination of asource of electrical musical tone signals including substantially oddharmonics only and having steep Wave fronts, a signal transmisisonapparatus connecting the source with the output system includingdiierentiating and reotifying means and a iilter, the differentiatingand rectifying means providing sharp pulses of only one polarity and thelter attenuating the harmonics in the pulses approximately directly inproportion to the frequencies of the harmonics, whereby musicallydesirable signals having both even and odd harmonic partials aresupplied to the output system.

JOHN M. HANERT.

i4 REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Darte 2,276,390 Hanert Mar. 17, 19422,340,001 McVellp Jan. 25, 1944 2,403,090 Larsen July 2, 1946 2,410,883Larsen et a1 Nov. 12, 1946

